This invention relates generally to optical focusing, aligning, and measuring systems and methods, and more specifically to such optical systems and methods for use in the fabrication of semiconductor integrated circuits.
Integrated circuits are electronic devices that are fabricated from a wafer of semiconductor material, such as silicon, by subjecting the wafer to a series of processing steps. The wafer, or substrate, is coated with a light-sensitive or photoresist material and then aligned with respect to a mask which allows light to expose the photoresist material in accordance with a predetermined circuit pattern. This pattern is subsequently used for developing circuit elements employed in forming the integrated circuits. Circuit patterns are thus transferred to the wafer photolithographically, with several different masks being used to form a typical integrated circuit. Successive wafer alignments with respect to the masks are critical to insure functional integrated circuits.
Proximity or out-of-contact printing, where the mask and the wafer are separated by a small gap (typically in the ten micrometer to fifty micrometer range), is a popular integrated circuit fabrication technique. Process yields using proximity or out-of-contact printing are better than those attainable using contact printing because of reduced mask impairment and improved mask lifetime due to the absence of contact between the mask and the wafer during printing. The proximity or out-of-contact method of printing is also the only method that can be used in X-ray lithography.
Proximity or out-of-contact printing is not without problems, however, because the gap separating the mask and the wafer must be great enough to allow for the surface flatness tolerances in both the mask and the wafer, but small enough to minimize diffraction effects on the projected circuit pattern. Integrated circuits are commonly designed with circuit pattern details which require alignment tolerances in the submicrometer range. Magnification optics of 100.times. to 1000.times. are commonly used in aligning the wafer with respect to the mask to resolve the alignment to the required tolerances, but such high magnification optics with high numerical apertures have a depth of field that is less than the gap separating the mask and the wafer. This means that either the mask or the wafer, but not both, can be in focus at any one time, thereby causing great difficulty in positioning the wafer for proper alignment with respect to the mask.
Thus, an optical system is needed that is capable of simultaneously displaying to an operator a magnified and focused view of both a mask and an out-of-contact wafer to facilitate mask-wafer alignment. A measurement system is further needed that is capable of measuring and adjusting the gap separating the mask and the wafer to a value suitable for proximity or out-of-contact printing. In addition, an alignment system is needed that is capable of more precisely aligning the wafer with respect to the mask.
Optical methods for focusing on two planes separated by more than the depth of field of the optics exist in the prior art. U.S. Pat. Nos. 3,488,104, 3,709,579, and 3,990,798 all disclose dual focus apparatus utilizing optical compensators that are fabricated for a specific gap value. However, these compensators are not easily adjustable to accomodate a range of gap values.
A gap measurement device is shown in U.S. Pat. No. 4,165,178, but this device is a go/no-go optical gauge that is limited to sensing separation distance at edges only. The use of a single moveable converging lens to measure the gap between a mask and a wafer is described in U.S. Pat. No. 4,070,116. However, careful study of this patent reveals problems that are inherent in the use of a single moveable converging lens for gap measurement.
The governing optics equation states that the reciprocal of the focal length of a lens equals the sum of the reciprocals of the object distance and the image distance. Since the focal length of a lens is constant, an increase in the image distance is balanced by a decrease in the object distance. In U.S. Pat. No. 4,070,116 the moveable lens is employed with a fixed focus detector to measure the gap between the mask and the wafer. After focusing an image of the mask onto the focus detector, the lens is moved away from the focus detector to a position at which the lens focuses an image of the wafer onto the focus detector. This displacement or change of position of the lens increases the image distance from the lens to the focus detector and causes a corresponding decrease in the object distance from the lens to the object plane. Due to this decrease in the object distance, the lens displacement must be greater than the gap between the mask and the wafer. The gap measurement calculation described in U.S. Pat. No. 4,070,116 therefore is quite complex and requires knowledge of the mask image distance and the mask object distance as well as the lens displacement. Moreover, U.S. Pat. No. 4,070,116 discloses no provision for establishing measurement accuracy greater than the depth of field of the optics.
Focus detectors sensing light intensity variations are also known to exist in the prior art. See, for example, U.S. Pat. Nos. 3,356,854 and 4,070,116, which show optical fibers attached to photodetectors and signal conditioning circuitry.